The invention relates to a method of installing components in a space defined between a cylindrical outer case of apparatus placed down an oil or gas well and a cylindrical inner passage passing through the apparatus.
Throughout this text, the term xe2x80x9ccomponentxe2x80x9d designates any type of electrical or electronic circuit be it simple or complex, integrated or otherwise, isolated or associated with other items, and also other components such as sensors, motors, etc. . . . .
The invention also relates to a downhole apparatus including components installed by the method.
In apparatuses that are to stay permanently down a well that is in production, an ever-increasing number of electrical devices such as motors, sensors, actuators, etc. are being used. A consequence of that is to increase the size of the components that need to be installed in such apparatuses.
In the past, when the components were small in size, they could be installed without difficulty in small boxes fixed along the production column defining the cylindrical inner passage of which the hydrocarbon rises. Such an arrangement has been in use for many years, in particular for installing pressure sensors on apparatuses that are designed to remain permanently downhole or on test apparatuses associated with drilling strings.
Because of the ever increasing complexity of components, that type of installation is generally no longer usable since it is unsuited to the largest sizes of such components.
A first conventional solution for solving this new requirement is shown diagrammatically in cross-section in FIG. 1 of the accompanying drawings.
In that case, the components 1 are installed in an annular space 2 formed between a tubular outer case 3 and a tubular inner production column 4 defining internally a cylindrical inside passage 5 up which hydrocarbon rises. In that arrangement, used in particular for test apparatuses installed on drilling strings, the outer case 3 and the production column 4 are arranged coaxially.
The conventional arrangement shown in FIG. 1 is generally satisfactory. Nevertheless, it can only be used when the ratio between the diameter of the outer case 3 and the diameter of the inner production column 4 is sufficiently large to define an annular space 2 between them that is capable of housing all of the components 1 that are to be installed in the apparatus under consideration.
When the above-mentioned diameter ratio becomes too small, it is conventional to use a second installation technique which is shown diagrammatically in cross-section in FIG. 2 of the accompanying drawings.
In that arrangement, the small ratio of the diameters between the tubular outer case of the device and the cylindrical inner passage 5 is compensated by locating the passage eccentrically. In general, the components 1 are then installed in recesses 6 machined externally in a solid metal core 7. Once the components 1 have been mounted in the recesses 6, the recesses are closed in sealed manner by covers 8 which are either welded at 9 to the core 7, or else are closed by a tubular sheath surrounding the core.
That conventional arrangement suffers from numerous drawbacks.
Thus, because of the large radius of curvature of the outer case of the apparatus, the covers 8 or the sheath need to be very thick in order to be capable of withstanding the large pressure difference that exists between the external downhole pressure (frequently 1000 bars to 1500 bars) and the internal atmospheric pressure. The large thickness of the covers or of the sheath significantly reduces the amount of space available for the components, and that goes against the desired objective.
Given the sometimes very large dimensions of the recesses 6, and in order to avoid excessively increasing the thickness of the covers 8 or of the sheath, use is sometimes made of support props 10 interposed between the cover 8 or the sheath and the bottom of a recess 6. In order to be effective, such support props must be of large section and they must be relatively close together, thereby likewise reducing the space available for components and requiring modifications to their shapes and above all to their interconnections when they are electronic components. In addition, the presence of support props 10 does not prevent the covers 8 or the sheath from sagging under the effect of the pressure difference in those portions of the covers that are not provided with support props.
When covers are used to close the recesses, another major problem that arises with the conventional arrangement as shown in FIG. 2 lies in obtaining leakproof sealing between the covers 8 and the core 7, since the sealing must be capable of withstanding the large difference between the external pressure downhole and the atmospheric pressure that exists inside the recesses 6.
Thus, achieving such sealing by means of elastomer gaskets is not recommended for use of long duration because such gaskets age. Furthermore, metal-on-metal sealing is difficult to design.
The only technique that can guarantee the desired degree of sealing in the long term is therefore welding. Because of the shapes of the covers, TIG welding is generally preferred over electron beam welding. Nevertheless, that type of welding suffers from the drawback of heating the components and, with some materials, of giving rise to stresses in the steel used for making the core 7 and the covers 8 or the sheath. These stresses can be relaxed only by subsequent heat treatment that certain components are incapable of withstanding. Such an operation is therefore not performed. As a result there is a significant increase in the risk of the core corroding.
Furthermore, for reasons of reliability, some components such as electronic circuits must be maintained as far as possible in an environment that is clean, in particular during manufacture and assembly. When the arrangement shown in FIG. 2 is used, it is very difficult to satisfy this condition during the step of mounting the circuits. Welding the covers 8 onto the core 7 by means of the TIG technique is an operation that is lengthy, and that takes place in a workshop in an environment that is ill-suited to protecting components.
The way in which the covers 8 are fixed onto the core 7 also makes repairing or replacing the components completely impossible. If a component fails, the core must necessarily be returned to the workshop to be cut up.
Other drawbacks of the technique shown in FIG. 2 stem in particular from using a core 7 that is in a single piece. That characteristic gives rise to high raw materials and machining costs and to machining operations that are complex and that require, for example, a certain number of bores to be made such as the bore referenced 11 to enable electrical link conductors to pass between components installed in different recesses 6. The complexity of the machining applied to the core gives rise to a non-negligible risk of it being necessary to remake the piece completely in the event of an error occurring during machining.
Furthermore, the presence of large internal volumes under atmospheric pressure inside the recesses 6 makes it necessary to use high quality alloys so as to withstand the pressure difference. Such alloys are more difficult to machine and above all they are more expensive than traditional alloys.
Finally, because the components 1 are mounted directly on the core 7 which withstands the mechanical forces applied to the production column (tension/compression, twisting, thermal expansion), it is necessary to decouple the components mechanically from the core.
A particular object of the invention is to provide a method enabling components to be installed in an original manner in downhole apparatus, particularly when the ratio of inside to outside diameters is too small to allow annular installation of the type described above with reference to FIG. 1.
More precisely, the invention provides a method of installing components in a space defined between an outer cylindrical case of downhole apparatus and a cylindrical passage passing through the apparatus in its longitudinal direction, the method being characterized in that it consists in mounting said components in a plurality of sealed tubes disposed in said space, one end of each tube being fixed to an interconnection box that is also placed in said space.
The installation method defined in this way solves all of the problems posed by the conventional techniques described above with reference to FIGS. 1 and 2.
Thus, by mounting the components in tubes of relatively small diameter it is possible to use walls of small thickness, thereby releasing a maximum amount of volume for the components. In addition, the preferably cylindrical shape of these tubes makes it possible to weld them at respective first ends to the interconnection box and to close them at their opposite ends by means of welded plugs. Reliable and effective sealing is thus obtained without any risk of damaging the components.
Furthermore, it is easy to preserve a clean environment for the components during assembly. Once they have been inserted into the tubes, which can be done in a clean room provided for this purpose, the tubes can be closed immediately in sealed manner by screwing the plugs into place with interposed sealing gaskets. The plugs can subsequently be welded in a workshop designed for that purpose without the clean environment of the components being affected.
The installation method of the invention also makes it possible to interconnect the components by means of electrical conductors which pass through the inter-connection box. Complex machining and sealing is thus avoided.
Mounting the components in tubes also makes it possible to eliminate any solid pieces and the lengthy and complex machining that would be associated therewith. This makes it possible to dissociate the electrical portions completely from the mechanical portions. In addition, this form of assembly reduces raw materials costs and machining costs and makes it possible to perform maintenance or component replacement operations, should that be necessary. Advantageously, the method of the invention is applied to the situation where the cylindrical passage is eccentric relative to the outer case of the apparatus.
The invention also provides downhole apparatus comprising a cylindrical outer case with a cylindrical passage passing therethrough in a longitudinal direction, and components mounted in a space defined between the case and the passage, the apparatus being characterized in that it further comprises a plurality of sealed tubes disposed in said space and in which the components are housed, and an interconnection box also disposed in said space and to which one end of each tube is fixed.
Preferably, the tubes extend in said longitudinal direction and they are advantageously fixed to the inter-connection box by circular welds.
Similarly, each plug is preferably fixed to one of the tubes by a circular weld and is advantageously fitted in an open end of the tube with an interposed sealing gasket.